M. J. S. Lowe

On the development and testing of a guided ultrasonic wave array for structural integrity monitoring

The prototype of a guided ultrasonic wave array for the structural integrity monitoring of large, plate-like structures has been designed, built, and tested. The development of suitably small transducers for the excitation and measurement of the first antisymmetric Lamb wave mode A/sub 0/ is described. The array design consists of a ring of 32 transducers, permanently bonded to the structure with a protective membrane, in a compact housing with the necessary multiplexing electronics. Using a phased addition algorithm with dispersion compensation and deconvolution in the wavenumber domain, a good dynamic range can be achieved with a limited number of transducers. Limitations in the transducer design and manufacture restricted the overall dynamic range achieved to 27 dB. Laboratory measurements for a steel plate containing various defects have been performed. The results for standard defects are compared to theoretical predictions and the sensitivity of the array device for defect detection has been established. Simulated corrosion pitting and a defect cut with an angle grinder simulating general corrosion were detected.

The Effect of Bends on the Propagation of Guided Waves in Pipes

The practical testing of pipes in a pipe network has shown that there are issues concerning the propagation of ultrasonic guided waves through bends. It is therefore desirable to improve the understanding of the reflection and transmission characteristics of the bend. First, the dispersion curves for toroidal structures have been calculated using a finite element method, as there is no available analytical solution. Then the factors affecting the transmission and reflection behavior have been identified by studying a straight-curved-straight structure both numerically and experimentally. The frequency dependent transmission behavior obtained is explained in terms of the modes propagating in the straight and curved sections of the pipe.

Short range scattering of the fundamental shear horizontal guided wave mode normally incident at a through-thickness crack in an isotropic plate.

Interaction of the fundamental shear horizontal mode with through-thickness cracks in an isotropic plate is studied in the context of low frequency array imaging for ultrasonic guided wave nondestructive evaluation with improved resolution. Circular wave fronts are used and the symmetric case where a line from the wave source bisects the crack face normally is considered. Finite element simulations are employed to obtain trends subject to analytical and experimental validation. The influence of the crack length and of the location of source and measurement positions on the specular reflection from the crack face is first examined. These studies show that low frequency short range scattering is strongly affected by diffraction phenomena, leading to focusing of energy by the crack in the backscatter direction. Study of the diffraction from the crack edges reveals contributions due to a direct diffraction at the edges and multiple reverberations across the crack length. A simple diffraction model is shown to adequately represent cracks up to moderate lengths, providing an easy means of estimating the far field of the waves. The presence of multiple diffraction components is quantitatively established and surface waves on the crack face are identified as equivalent to low frequency symmetric modes of rectangular ridge waveguides.

Torsional waves propagation along a waveguide of arbitrary cross section immersed in a perfect fluid

Guided torsional waves in a bar with a noncircular cross section have been exploited by previous researchers to measure the density of fluids. However, due to the complexity of the wave behavior in the noncircular cross-sectional shape, the previous theory can only provide an approximate prediction; thus the accuracy of the measurement has been compromised. In this paper, a semianalytical finite element method is developed to model accurately the propagation velocity and leakage of guided waves along an immersed waveguide with arbitrary noncircular cross section. An accurate inverse model is then provided to measure the density of the fluid by measuring the change of the torsional wave speed. Experimental results obtained with a rectangular bar in a range of fluids show very good agreement with the theoretical predictions. Finally, the potentials to use the model for sensor optimization are discussed.

Material property measurement using the quasi-Scholte mode—A waveguide sensor

In the food industry and other industries, rheological measurements and determination of particle sizes in suspensions and emulsions is of great importance for process and quality control. Current test cell based ultrasonic methods exist but are often inconvenient. An attractive alternative could be to insert a simple measurement “dipstick” into the fluid; this paper presents an initial study of the feasibility of using measurements of the velocity and attenuation of the quasi-Scholte mode on a plate to obtain the longitudinal velocity and attenuation of an embedding medium. The attenuation of the quasi-Scholte mode is caused by two mechanisms: shear leakage and attenuation due to the bulk longitudinal attenuation of the embedding material. In a calibration test the bulk longitudinal velocity and viscosity of glycerol were determined experimentally. Measurements agreed well with results from conventional methods and literature data. Quantitative results and an independent validation for honey, a very visc...

Scattering of the fundamental torsional mode by an axisymmetric layer inside a pipe

The scattering of the fundamental guided torsional mode by a local axisymmetrical layer coated inside a pipe, referred to as a bilayered pipe, is studied. In a prescribed frequency range, the number of torsional modes which can propagate in an empty pipe is increased by the presence of the coating layer, including new cutoff frequencies that depend on the layer thickness and shear acoustic properties. This principle suggests the potential for detecting, and perhaps characterizing layers inside pipes, which may be exploited in two ways using either remote or local measurements. A remote measurement may be employed to measure the reflection from the entry point of the layer inside the pipe. Such a measurement shows that the reflection coefficient spectrum exhibits periodic maxima that occur at the cutoff frequencies of the torsional modes in the bilayered pipe. On the other hand, when the location of the coating layer is accessible, the local guided wave measurement can be made to measure the dispersion cur...

The scattering of guided waves in partly embedded cylindrical structures.

The scattering of ultrasonic guided waves at a point where a free cylindrical waveguide enters an embedding material is investigated. A modal solution that is valid when the guided waves are incident from the free section of the waveguide is developed. It is shown that in this case it is valid to consider only the modal fields over the cross section of the waveguide, neglecting the fields in the embedding material. As an application, the scattering of the lowest-order longitudinal mode in a cylindrical waveguide, L(0,1), is examined in detail. As well as considering epoxy resin as an embedding material, the case where the embedding material is replaced by a perfectly rigid boundary is discussed. The latter gives some insight into the role of nonpropagating and inhomogeneous waveguide modes in the scattering process. The results from the modal solution are validated using Finite Element modeling, very good agreement being obtained.

Efficient Numerical Modelling of Absorbing Regions for Boundaries Of Guided Waves Problems

This paper presents a method of modelling guided waves propagation with Finite Elements using frequency domain analysis and absorbing regions. These regions are placed at the extremities of the models and can absorb guided waves, thus avoiding reflections from the boundaries. Guidelines for efficient definition of these regions are discussed. Typical case studies are presented in order to validate the process. This method will benefit the modelling of large problems such as the interaction of guided waves with complex 3D defects in complex 3D structures.

Review of Synthetically Focused Guided Wave Imaging Techniques With Application to Defect Sizing

Synthetically focused imaging has been used for some time in the NDE community. The techniques have primarily been directed towards imaging using bulk waves. There has recently been use of SAFT (Synthetic Aperture Focusing Technique) using guided waves in plates. Here, we review three different synthetically focused imaging algorithms for a linear array aperture: CSM (Common Source Method), SAFT and TFM (Total Focusing Method). The resolution of the different techniques is obtained from scalar diffraction theory and then validated by means of a low frequency (50kHz) steel plate experiment using PZT excitation and laser reception of the A0 mode. Imaging of through thickness slits parallel to the array is then discussed.

Feasibility study of sludge and blockage detection inside pipes using guided torsional waves

The accumulation of sludge and blockages in pipes is a problem which affects many industries. It has been previously reported that in principle sludge and blockages can be detected and even characterized by using guided ultrasonic torsional waves, based on an idealized model in which the sludge layer was simplified in terms of geometry and material properties. The work revealed that the presence of a layer inside a pipe scatters the guided wave propagating in the pipe and both the reflection and transmission of the guided wave can be used to effectively detect and characterize the layer. Accordingly, two guided wave measurement techniques have been proposed. This paper proceeds the work by taking into account more realistic sludge characteristics, including irregular axial and circumferential profiles of the sludge layer, imperfect bonding state between the sludge and the pipe and the material damping of the sludge. The influence of these issues is investigated to identify the critical factors that influence the detection and characterization capability of the two measurements. The study shows that both reflection and transmission measurements can be exploited usefully and non-intrusively to detect realistic accumulations of sludge and blockages; however, the quantification of such materials will be difficult due to their arbitrary shape and properties.